Method of making filter paper



Patented Jan. 14, 1941 UNITED STATES PATENT OFFICE METHOD OF MAKINGFILTER PAPER tion of Delaware No Drawing. Application October 8, 1937,Serial No. 168,082

4 Claims. (01. 210-204 This invention relates to filter paper and moreparticularly to filter paper for use under conditions where it isnecessary to combine a high rate of flow of filtrate per square inch ofarea with substantial tensile strength and stiffness.

An object of the invention is to produce a paper combining largecapacity per unit of area, high tensile strength, and good capacity forremoving fine particles of solid matter.

A further object of the invention is to produce a filter paper of theabove character which is capable of being operated at high fiuidpressure without having the fibres of the paper matted together by thepressure in such a way as to retard the flow of filtrate.

A further object of the invention is to produce a filter paper which iscapable of retaining the above described characteristics in the presenceof water or dampness in the filter and which will, go in fact, becapable of filtering water Without loss of tensile strength or any ofthe other qualities above described.

In making filter paper-according to our invention, we may use ordinarywood pulp paper or paper made from cotton rags or paper made from woolfibre. We may also use any paper containing these or similar ingredientsor mixtures thereof. These papers are well known on the market and nospecified description is necessary. The thickness of the paper may beselected according to the purpose for which it is intended, but anapproximate thickness of has been found to be satisfactory for mostpurposes.

In preparing filter paper according to our invention, we first take thedried and rolled porous paper stock as furnished by the paper mills andsoak it in a solution of caustic soda (NaOH) in water. Any desiredstrength of solution may be used, but a 5% solution is suggested.Stronger 40 solutions may also be used, particularly in cases where itis desired to use the same solution a second time after it has beenwashed out of the paper. The washing process naturally dilutes thesolution, and by starting out with a strong solution, it is possible touse it several times before it falls below its strength sufficient togive satisfactory results. When the solution becomes too Weak, it may,in most instances, be brought up to strength by boiling as long as thechemical composition of the caustic soda has not been destroyed.

The temperature of the soda bath need not be held within accurate limitsand ordinary room temperature F.) is satisfactory. The time controlledby the operator who observes the paper in the soaking and swellingprocess. From one to five minutes in the soaking bath is usuallysufficient. After soaking, the paper will be found to be swelled toapproximately twice its normal thickness and the length and width of thesheet will be shrunk. The paper is now in a very much weakenedcondition, not being much more than a mush, so that it is necessary tohandle it on or between screens to keep it from falling apart. After thesoaking process, the caustic soda solution is washed out with water, thefirst part of the Wash water being saved for reuse. 'As soon as thecaustic is removed from the paper, its strength is substantiallyincreased, but it is still not strong enough to stand rough handlinguntil it is dried. The drying is preferably in the presence of heat.

When the paper has been thoroughly dried, its thickness will be found tobe substantially increased, although the length and width of the sheetwill be smaller than the original. The paper is now in condition for useas a filter, and the fiow capacity per square inch will be substantiallyincreased over the original. We do not, however, consider that eitherthis product or the original paper is entirely suitable for filteringfluids containing Water or dampness because of the great loss of tensilestrength in the presence of water. In order to make the paper thoroughlysatisfactory, we give it a further treatment with a 10% lacquer solutionas described in the, United States patents to G. R. Ericson #1,944,550,issued January 23, 1934, and #2,007,753, issued July 9, 1935. After thistreatment, the paper will be found to have its increased flow capacityand will have greater tensile strength and resistance to moisture. Thelacquer solution may be made weaker or stronger as'desired, but amixture of 10% of ordinary brushing lacquer and thinner gives highlysatisfactory results. For use in filtering liquids containing gasoline,we prefer to use a lacquer containing glyptal gum or other substancesimilarly insoluble in gasoline.

The operation above described first has the effect of loosening,distorting or rearranging, and swelling the fibres to at least twicetheir normal size which shrinks the paper in the plane of the sheet andsubstantially thickens it. This operation also expands the intersticesbetween the fibres whereby a relatively hard paper may be used and thefiltering capacity of the paper is thereby increased. After the caustictreatment, the interstices will be more tortuous which results in betterfiltering properties than would be the case if a more loosely matted orsofter paper were used in the first place. The lacquer coating has theeffect of strengthening, bonding, :and fixing the distorted fibres andrendering the paper resistant to gasoline and moisture. The resultantproduct is an eflicient, durable filter paper adapted for use,particularly, in the fuel supply lines of internal combustion engines.

The exclusive use of all modifications as come within the scope of theappended claims is contemplated.

We claim:

1. The method of conditioning a sheet of porous paper stock for use asfilter paper which comprises loosening and distorting the fibres toincrease the filtering capacity of the stock and applying a thinsolution of glyptal lacquer to simul taneously water-proof fibres andcause them to adhere to each other.

2. A filter paper comprising a sheet of porous, paper stock in which thefibres are loosened and distorted and provided with a bonding and liquidresistant coating.

3. The method of conditioning filter paper which comprises firstswelling the fibres by soaking the paper in a solution of caustic soda,washing out the caustic solution after the fibres have been suflicientlyswelled, drying the paper, applying a thin solution of lacquer, andagain drying.

4. A filter paper comprising a mat of fibres, said fibres having beenloosened and distorted by a swelling and reshrinking, and being cementedtogether at their points of contact and provided with a water resistantcoating.

W. C. MAGRUDER, JR. A. C. KORTE.

